There are a variety of applications that require the use of a line image having a relatively uniform intensity. One such application is laser thermal processing (LTP), also referred to in the art as laser spike annealing (LSA), or just “laser annealing.” Laser annealing is used in semiconductor manufacturing for a variety of applications, including for activating dopants in select regions of devices (structures) formed in a semiconductor wafer when forming active microcircuits such as transistors and related types of semiconductor features.
One type of laser annealing uses a scanned line image from a light beam to heat the surface of the wafer to a temperature (the “annealing temperature”) for a time long enough to activate the dopants in the semiconductor structures (e.g., source and drain regions) but short enough to prevent substantial dopant diffusion. The time that the wafer surface is at the annealing temperature is determined by the power density of the line image, as well as by the line-image width divided by the velocity at which the line image is scanned (the “scan velocity”).
To achieve high wafer throughput in a commercial laser annealing system, the line image should be as long as possible, while also having a high power density. An example range for usable line-image dimensions is 5 mm to 100 mm in length (cross-scan direction) and 25 microns to 500 microns in width (scan direction), with typical dimensions being 10 mm long by 100 microns wide. To achieve uniform annealing, it is also necessary for the intensity profile along the line-image length to be as uniform as possible, while non-uniformities along the line-image width are averaged out during the scanning process.
Typical semiconductor processing requirements call for the annealing temperature to be between 1000° C. and 1300° C., with a temperature uniformity of +/−3° C. To achieve this degree of temperature uniformity, the line image formed by the annealing light beam needs to have a relatively uniform intensity in the cross-scan direction, which under most conditions is within +/−5%.
Typical semiconductor applications require an annealing time of 0.1 milliseconds to 10 milliseconds (ms). To meet this requirement, a mechanical stage can be used to move the wafer perpendicular to the long dimension of the beam. With a stage velocity of 100 mm/sec and a short beam width of 100 microns, the thermal annealing (dwell) time is 1 ms.
Unfortunately, for certain semiconductor device fabrication situations, the annealing temperature and annealing time are constrained by other factors, such as wafer thickness and the type of semiconductor device structures formed on the wafer. In such situations, the conventional annealing (dwell) times provided by conventional laser annealing systems are unsuitable.